This is probably "Mutiny" (1980) by Malcolm Hulke. It was only published in an anthology called either Galactic Adventures or Purnell's Book of Adventures in Space. (I'm not sure they're actually 2 distinct books, because they have the exact same content, down to the cover and interior art and even the pagination is the same, and were both published in 1980.)
Synopsis:
An Earth ship with a small crew has just defeated an enemy spaceship. The crew, Baalto specifically, are complaining about not having had a leave in two years. The navigator, Xoc, notes that there is a suitably Earth-like planet just two days travel away, but when asked by Lieutenant Hoddre, Captain Naek instead orders the ship to travel in the opposite direction.
This pushes the crew, especially Baalto, to the breaking point and he takes the captain at gunpoint. He wants to kill the captain, but the captain points out that the ship's computer will only respond to his voice. The crew forces the captain to order the ship to return to Earth, a journey that takes a month.
In Earth orbit the crew discovers that the enemy has devastated Earth; there are apparently no survivors. The captain reveals he has known this for a year and chose to keep fighting to exact some revenge on the enemy. Baalto is barely kept from killing the captain immediately, because the crew do not wish to be stranded in orbit around a dead planet.
Frith has the idea to make the captain record the necessary commands to travel to the planet and land there, so they can kill him and still make the trip. Melos suggests that they test it first, which works, and the commands are recorded.
With the captain spaced, it rapidly becomes apparent that the captain's habit of always saying "please" to the ship's computer wasn't simple politeness, it was a necessary part of the orders. Since the captain only recorded the test order with "please," none of the other orders required to travel to the planet will work. The crew is stranded in Earth orbit forever, floating alongside the dead body of the captain.
About the specific points you recall; the computer has legs:
Captain Naek switched off. He turned to the ship's computer, a small, almost featureless black box mounted on spindly legs and wheels. "Please, did you hear that?"
They test a simple order:
"Good idea." Baalto switched on the captain's own tape recorder. "Speak into
this and tell the computer to speed up its present orbit around Earth."
[...]
Order heard and understood, said the computer.
Xoc looked out from one of the portholes. "It's working. We're travelling
much faster."
The computer doesn't respond to the recorded orders:
Xoc had the tape recorder ready. "This first message is to break us out of Earth's orbit." He pressed the button.
Accelerate to hyperdrive, said the captain's recorded voice. Leave Earth's orbit in the direction of the Moon.
The computer was silent. Xoc put the recorder on rewind and played the message again. Nothing happened.
They realize that the commands need to be said with "please" to be accepted:
Melos suddenly threw his head into his hand. "Please," he said quietly. The others looked at him, not understanding.
"The voice-print," said Melos, "The computer only followed instructions that began with the captain's voice saying please. That's why he was always polite to the computer. And we just erased all his voice-prints from the tape. He's fooled us. We're going to stay in orbit round Earth forever."
